The invention relates to the compression of a colour encoded picture using digital techniques including storage means as for example a frame store.
Digital storage techniques are well known in the television art as in time base correctors (e.g. see British Pat. No. 1,465,314 and U.S. Pat. No. 3,978,519). If the store is greatly expanded to allow a whole frame to be captured (i.e. a frame store synchroniser) then the store control must also be expanded to handle the necessary picture point addressing and sequencing. A frame store is disclosed in British patent application No. 6585/76 and U.S. patent application Ser. No. 764,148 A synchroniser incorporating a frame store is described in British patent application No. 6588/76 (U.S. Patent Application Ser. No. 769,615, now U.S. Pat. No. 4,101,939).
If a monochrome system is devised where a digitally encoded signal is read into a framestone that is capable of asynchronous action reading its content to a display at a rate dictated by the display timing waveforms, and if, on the input, the addresses were so modified that every alternate picture point along the line the address was not advanced, the net result would be that after a complete line only half of the normal store allocations for this line will have been filled. The output examining the contents of the store in the normal way, will therefore see an image compressed horizontally to half its original size.
If the same is done in the line direction so that the line address counters are only advanced every alternate line, the result will be a picture of half the original height. Performing these two functions together therefore creates a quarter size picture.
Whilst in monochrome this technique will look quite good there will be a number of flaws, firstly in the horizontal direction a very thin vertical line that occupied a picture point that the address counter has ignored will not be present on the output picture. However, if the camera pans sideways, this very thin line will alternately appear and disappear as it moves across cells that have been examined by the input circuitry and cells that have been ignored. This problem can be overcome on the input by including a suitable filter that has a time constant equal to two picture points. In this way vertical lines on the picture that are only one picture point wide in fact will appear at half amplitude but will not twinkle as the camera is panned.
The problem in the vertical direction is more complex to overcome but can be achieved by a line interpolator effectively adding one line to the next and dividing by two prior to entering the store. Using this technique it can be demonstrated that a seemingly perfect quarter sized compressed picture can be generated. Furthermore the technique can be extended to include variable sizes between full size and any tiny picture.
When attempting to operate with encoded colour, realisation of a suitable compressor becomes more difficult. Consider in the horizontal direction what will happen if `one picture point on -- one picture point off` were used. The normal sampling frequency for NTSC colour is three times colour sub-carrier and, therefore, taking `one picture point on -- one picture point off` would completely alter or destroy the sub-information. A technique that maintains the hue correct and does not corrupt the sub-carrier is, instead of entering one picture point into the store and ignoring the next, is to enter three consecutive picture points and to ignore the next three consecutive points. This results in quarter size pictures, as before and gives the correct hue but suffers very badly from problems associated with interpolation since now a line three picture points wide will appear and disappear as it is moved across the input field of view. Avoiding this effect is difficult and although interpolation is possible the result is a picture of greatly reduced bandwidth.
The problem in the vertical direction is similar since the phase of sub-carrier is inverted line to line, if sub-carrier phase is to be maintained correctly then two consecutive lines must be used, the next two consecutive lines ignored and so on. The net result of the vertical and horizontal operation is a picture that is quite unacceptable containing what appears to look like cells, that twinkle as the camera is panned, and sloping edges on the picture appear with a staircase type effect.